Note: Descriptions are shown in the official language in which they were submitted.
l:'iL~6B~;~.3
PHD. 81094
m e invention relates to an X-ray generator for an X-ray tube
comprising a grounded grid which is situated between the anode and the
cathode thereof, said X-ray generator ccmp~ising a series connection
of high voltage generators which are to be connected to the anode and
the cathode of the X-ray tube in order to supply the X-ray tube with a
direct voltage, and also comprising means for changing the ratio of the
anode voltage and the cathode voltage.
An X-ray generator of this kind.is known from ~E-OS 29 17 636.
An example of X-ray tubes to be pcwered by the X-ray generator
in accordance with the invention.is known from the magazine "ME3IC~MW~DI",
Vol. 25, No. 1, 1980, pages 29 and 30 and fro~ DE-OS 28 50 583. X-ray
tubes of this kind are also available from Philips under the name
"Super Rotalix Ceramic"*~ m ey deviate frcm the customary grid con-
trolled X-ray tubes in that the voltage on the metal grid is.usually
positive with respect to the cathode and can:assume very high values
which correspo~d to half the maximu~ tube voltage or even more. In this
type of tube the anode current;is smaller than the cathode current,
because part of the electrons is inci~ent.on the grid after reflection
from the anode. The electron current.arriving on the anode in such an
X-ray tube:is determined not.only by the cathode temperature but also to
a high degree by the ~oltage hetween the cathode and the grounded grid;
conse~uently the cuxrent ~hrough the X-xay tube decreases when the
cathode voltage decreases.and.the cathode temperature remains constant,
with the result that full e~ission cannot be attained. When use.is
made of high voltage generators hav..ing a very high internal resistance,
for example, generators compLising d.c./a c. converters, there is addi-
tional problem.in that the cathode ~oltage decreases more than the anode
voltage ~hen the tube current m creases, because the anode current is
smaller than the cathode current. Ev.en when the voltage between a~ode
and cathode is maintained ~ a cons~ant v~lue by mea~s of a suitable
con~rol circuit, thç cathode voltage per se will decrease and hence
also the emission current.
In the X~ray generator disclosed in DE-OS 29 17 636 thLis
*~egistered Trademaxk
6~
P~ 81094 2 17.08.1982
effect is eliminated in that the high voltage generator,which is con-
nected in series with the high voltage generator on the anode side
via a grid-controlled X-ray tube,generates a voltage which is higher
than that generated by the high voltage generator on the anode side.
ThR additional steps required for this solution (another grid-controlled
tube), however, are very expensive.
It is the object of the invention to provide an X-ray genera-
tor of the kind set forth which prcduces a large emission current also
in the case of loW tube voltages, without elaborate additional facili-
ties being required.
To this end, the X-ray generator in accordance with the in-
vention is characterized in that the series connection consists of at
least three high voltage generators, there being provided a high voltage
switching device whereby an output junction of two interconnected high
voltage generators can be connectecl to ground, the high voltage swit-
ching device being switchable in dependence of the adjusted value
of the tube voltage and/or the tube current in order to ensure that the
ratio of the anode voltage and the cathode voltage is smaller in the
case of low values of the tube voltage than in the case of high values
of the tube voltage and that a given value of the cathode voltage is
not exceeded.
Thus, in accordance with the invention at least one of the
three high voltage generators is active on the side of the cathocle in
one position of the high voltage switching device and on the side of the
anode in the other position. ~7hen the relevant generator is active on
the side of the cathode (in the case of comparatively low cathocle or
tube voltage), the emission current is increased thereby. Ho~ever, if
this high voltage generator were also allo~.7ed to be active on the cathode
side in the case of high tu~e voltages (for example, 150 kV), particu-
larly in the case of low tube current settings a high voltage overload
could occur on the cathode side where, for example, 100 kV would be
present. Therefore, in the case of high tube voltages at which a high
voltage overload is liable to occur on the cathode side, this high
voltage generator should be active on the anocle side.
Thus, the only additional facility required is a high voltage
switching device. At least three generators must now be provided instead
of the customary two generators, but the generators may now be` construc-
ted for lower voltages. For e~ample, when each high voltage c~enerator is
~8~
P~ 81094 3 17.08.1982
formed hy a secondary winding of a high voltage transfonrer whose vol-
-tage is rectified by a rectifier device, three secondary windings are
required, but these windings may be at least partly constructed for a
voltage which is lower than if only high voltage generators were present.
The number of rectifier diodes c-~mprised in the rectifier devices is
not increased by the presence of three or more rectifier devices in
accordance with the invention, because the individual rectifier devices
may be constructed at least partly for lower voltages.
The invention will be described in detai] hereinafter with
reference to the drawings.
Figure 1 shows a simplified em~odiment, and
Eliqure 2 shows an embodiment comprising an emission current
measuring resistor.
The three high voltage generators comprise three secondary
lS windings 11, 21 and 31 of a high voltage transformer 4 whose primary
winding 5 is connected to a switching and control device (not sho~7n)
which enables the formation of voltages of predetermined value on the
secondary windings for a predetermined period of time. The secondary
windings 11, 21 and 31 form, in conjunction with a rectifier devi-ce 10,
20 and 30, respectively, a high voltage generator 1, 2 and 3, respective-
ly. The negative O~ltpUt terminal 12 of the high voltage generator 1
is connected to the positive terminal 23 of the high vfltage generator 2
whose negative output 22 is connected to the positive output 33 of the
third high voltage gererator 3. The positive output 13 of the high
voltage generator 1 and the negative output 32 of the high voltage
generator 3 are each connected, via a damping resistor 6, -to an X-ray
tube 7 which comprises a grounded metal grid 8 which is connected to
ground O between the anode and the cathode. The anode and the cathode of
the X-ray tube 7 are each connected to the ground O via a voltage divider
15 which serves for the measurement of the tube voltage. The tempera'clIre
of the filament 7' of the tube 7 is determined by the filament current
generated in a filament current transformer 16. Capacitors 17 and 17'
which are connected between the outputs 13 and 32, respectively, on the
one side and the ground O (ground potential) on the other side smooth
the voltage on the X -ray tube.
There is provided a high voltage switching device 9 which
connects, as desired either the negative output 22 of the high voltage
generator 2 or the negative output 12 of the high voltage generator 1
(whose potential is identical to the potential of the positive output
PHD 81.094 ~ 17.08.1982
of the high voltage generator 2) to the ground O. In the position of
the high voltage switching device 9 which is sho~n in -the drawing
the cathode voltage is produced only by the high v~ltage generator 3
whilst the anode voltage is produced by the high voltage ~enerators 1
and 2. In the second position (not shown) of the high voltage switching
device 9, however, the cathode voltage is produced b~ the high voltage
generators 2 and 3 together, whilst the anode voltage is produced only
by -the high voltage generator 1. In the second (latter) position, the
cathode voltage, therefore, is higher in comparison with the anode
voltage than in the position shown.
The sum of the output direct voltages of the high voltage
generators 1 and 2 should be equal to the output direct voltage of the
high voltage generator 3. It would thus be ensured,that in the position
of the high voltage switching device.which is shown~the tube voltage
is symmetrically distributed between the anode side and the cathode
side in the case of small tube currents (for example, during fluoros-
copy) and notably also in the case of high tube voltages. The ratio
of the output voltages of the high voltage generators 1 and 2, however,
should be proportioned in dependence of t.he interna] resistance of the
high voltage generat.or device 1, 2 and 3. The higher the internal
resistance thereof, the higher the output voltage of the high voltage
generator 2 must be in comparison with the output voltage of the high
voltage generator 1. When the primary winding 5 is connected to a d.c./a.c.
converter (not shown in the Figure), a comparatively hio,h internal
resistance arises, as is know~, for example, from that case the
secondary windings 11 and 21 preferably comprise the sam~ num~er of
turns and the rectifier devices 10 and 20 preferably comprise the same
numker of rectifier diodes, so that koth high voltage generators 1
and 2 supply eo,ually high output voltages. The output voltage of the
high voltage c~e~erator 3 should then be eq~lal to the sum of the output
voltages of the high voltage generators 1 and 2. It is particularly
attractive to form the high voltage generator 3 as the series connec-
tion of two high voltage generators which are identical to the high
voltage generators 1 and 2. In that case four identical high voltage
generators can be used, so -that the m~nufacture is cheaper.
Using the described proportioning of the high voltage generators
1, 2 and 3, the following modes of operation exist for the X-ray tube 7.
a) Small tube currents (some m~, for example, during fluorscopy).
In the position of the high voltage switching device 9 which is
PHD 81.094 5 17.08.1982
shown in Fi~lre 1, the anode voltage and the cathode voltage are
equal (disregarding the polarity). In the position of the high
vol-tage switching device 9 which is not shown, -the cathode voltage
will be three timeshigher than the anode voltage. In the case of
very high tube voltages, for example, 150 kV, this could cause an
excessive grid/cathode voltage difference, so tha-t a high voltage
overload for the X-ray tube 7 arises. Because in the case of (very)
small -tube currents a decrease of the tube current due to a de-
crease of the cathode voltage can also be compensated for by an
increase of the power to be dissipated in the filament 7' of the
X-ray tube, the switching device 9 will not be used in the e~ent of a
high voltage overload of the X-ray tube 7, so that the device is
continuously operated in the position of the high voltage switching
device 9 which is shown.
b) Large tube currents (a few hundreds of mA and higher, for example,
for radiographs).
~ ven though the no-load voltages on the output terminals 13
are 32 are equal in the position of the high voltage switching device
9 which is shown, the cathode voltage is lower than the anode vol-tage
in -the case of large tube currents. This effect is caused on the one
hand by the high internal resistance (as already explained) and on the
other hand by the fact that the cathode current is larger than the
anode current, because part of the electrons emitted by the cathode
is reflected to the grid 8 by the anode. Consequently, even in other-
wise the same circumstances (copper cross-section, number of turns,
etc), the voltage decrease on the cathode side is larger than that
on the anode side and the voltage distribution is asymmetrical. Notably
in the case of low tube voltages, the vo]tage on the cath~de is thus
liable to become so low that the desirable large tube ~lrrent cannot
flow. In that case the switching device 9 must be switched over to
the position which is not sho~n, the negative OlltpUt 12 of the high
voltage generator 1 or the positive outpu-t 23 of the high voltage
generator 2 then being connected to the ground O. The pronounced
asymmetry betw~en the anode voltage and the cathode voltage which
occurs in the case of very small tube currents is then partly compen-
sated for by the high internal resistance and the ineq~lality of the
cathode current and the anode current. In the case of very large
tube curren-ts, the value of the anode voltage and the cathode voltage
PE-~D 81.094 ~ 17.08.1982
m~ thus even kecome equal again. In that case) however, the tuke
current may be twice as large, in given circumstances even larger, than
the tube current which would occur for the same tube voltage and the same
filament tem~erature in the position of the high voltage switching device
9 which is shown in the drawing.When the internal resistance and the tube
current are so ~arge that a symmetrical voltage distribu-tion is obtained
on the X-ray tuke 7 in spite of asymmetrical no-load voltages, the high
switching device 9 may continuously remain in the indicated switching
position (not shown).
The high vo]tage switching device is contro~led by a control
device 18. The control device 18 serves to switch the high voltage
switching device 9 to the position which is shown in the drawing when-
ever the grid/cathode voltage difference would kecome so large that
in the other position of the high voltage switching device 9, considering
the given values of the internal resistance and the voltage oE the high
voltage generator 2 and the adjusted values of tuke voltage and tuke
current~a high volt~age overload would occur on the cathode side which
could cause, for example, breakdowns.
In the case of a comparatively low internal resistance of the
X-ray generator, the voltage distribution and the value of the cathode
voltage are substantially independent of the tube current. In such cases
it is sufficient to set the high voltage switching device 9 to the
position shown as soon as the tube voltage adjusted by the operator
exceeds a predetermined value. It is to ke noted that switching over
cannot be controlled in dependence of the smallest cathode voltage mea-
sured by ]ower measuring voltage divider 15, because in that case
switching -over ~uld have to take place in the presence of the tube
voltage, that is to say during an exposure or during fluorscopy; this
must be avoided. Switching-over should take place a]ready before the
selected tube voltage is switched on.
In high voltage generators having a very high internal
resistance, however, it may be -that in spite of asymmetrical no-load
vol-tage the tuke voltage is symmetrically distributed between the anode
side and the cathode side, so that in that case (large tube currents)
the high voltage switching device 9 may remain in the position which
is no,t shown. However, if only one very small tube current flows, the
voltage distrikution on the X-ray tube 7 also becomes asymmetrical,
so that switching-over is necessary. Therefore~switching over in depen-
dence of the adjusted tuke current then suffices.
~ 183~
PHD 31.094 7 17.08.1982
Generally, however, it is advantageous to perform the
swi-tching-over in dependence of the tube voltage and the -tube current.
To this end, the control device 18 com~rises a first switch 181 which
is coupled to the adjlstirg element 19 for the adjustment of the tube
vo]tage. rrhe switch 181 connects each time one end of one of four
resistors 182 (to the other end of which there is connected one of the
four voltages U1 ... U4) to a second switch 183 which can be switched
over to one of several resistors 184 of different value, the other end
of which is connected to ground. The voltages U1...U4 are proportional
to the vo]tage adjusted by means of the se]ector 19, whilst the resis-
tors 184 are approximately inversely proportional to the current
flowing via the grid 8 at the selected tube current, and hence also
to the tube current itself. The resistors 182 should correspond to
the internal resistance of the high voltage generator at-the associated
selected voltage. If the internal resistance is independent of this
voltage, the resistors 182 can be omitted if instead the voltage U1,
U2, U3 ard U4 applied to the first switch 181 are suppliecl by a direct
vo]tage generator having a corresponding internal resistance~
The voltage on -the connection between the two switches 181
and 183 is higher as the adjusted tube voltage is higher and ~s the
acljusted tube c~rrent is smaller. It is depenedent on the adjusted
values of tube current and tube voltage in the same~ay asthecatho~e vo~ge
(in the position of the high voltage switching device 9 which is not
shown) and is used for controlling the high voltage switching device 9.
For this purpose there is provided a comparator circuit 185 which
compares the voltage on the connection between the two switches 181
and 183 with a predetermined reference value UR and which switches the
high voltage switching device 9 to the position shown when the reference
value UR is exceeded, and switches it to the other posi-tion when this
voltage decreases below the reference value UR.
The control device 18 thus represents a simulation net~rk
which simula-tes the electrical conditions on the cathode of the X-ray
tube 7 (in the position of the high voltage switching device 9 which
is not shown). A simulation network of this kind may be omitted in X-ray
generators in which the values of tube voltage and tube curren-ts to be
adjusted are digital values and in which a programmable digital arith-
metic device is provided for control of the X-ray genexator. Instead
of using the si~mulation network, the cathcde voltage is calculated by
~e~
P~ 8l.094 8 17.08.1982
means of a program. The computer thus controls the high voltage switching
device 9 in dependence of the calculated cathode voltage.
Figure 2 shows an embodime~t comprising a measuring resistor 25
for measuring the tube current. It is known that the tu~e current of an
X-ray tu~e can be readily measured by utilizing a resistor 25 which car-
ries the tube current and one end of which is connected to ground O. The
tu~e current is the electroc current which reaches the anode where it ge-
nerates X-rays. In the described X-ray tubes, the cathode current is sub-
stantially equal to the tube current because the part of the electronswhich reaches the grid 8 is negligibly small for all practical purposes.
In the er~odiment shown in Figure 2, the meas~ring resistor 25 is connec-
ted between the two capacitors 17 and 17'. The high voltage switching de-
vice comprises four switching contacts 91 to 94. The swi-tching contact 91
connects the positive output 33 of the high voltage generator 3 either
to the negative output 22 of the high voltage generator 2 or to the junc-
tio~ 26 oE the resistor 25 and the capacitor 17. The switching contact
92 either connects the junction 26 near the resitor 25 to the positive
output 23 of the high voltage generator 2 or is open. The switching con-
tact 93 connects the end of the resistor 25 which is connected to ground Oeither to the negative ~utput 12 of the high voltage generator 1 or to the
positive output 23 of the high voltage generator 2. The switching contact
94 either connects the negative output 22 of the high vol-tage generator
2 to ground O or is open. All contacts 91 to 94 can be simultaneously
switched to the switching position which is sho~n or to the switching posi-
tion which is denotecl by broken lines b~ the control device 18 (figure 1).
which is not shown.in detail. In the switching position which is denoted
by broken lines, the current flows from the positive cu-tput 33 of the high
voltage genera-tor 3 to the negative output 22 of the high voltage genera-
tor 2 via the switching contact 91. From the positive output 23 of the ge-
nerator 2 the current flo~s to ground O via the switch 92 and the resis-
tor 25. From ground O, the current flows to the negative output 12
of the high voltage generator 1 via the switching contact 93. In this
position, the series-connected high voltage generators 2 and 3 to-
gether supply the cathode voltage whilst the high voltage generator 1supplies the anode voltage. In the switching position shcwn, the cur-
rent flows -to the junction 26 at one end of the resistor 25 via the
contact 91 and, via the resistor 25 and the switching contact 94, to the
negative,output 22 of the high voltage generator 2 whose positive
Pl-~ 81.094 9 17.08.1982
output 23 is connected to the negative output 12 of the high voltage
generator 1 via the switching contact 93. The negative output 22
of the high voltage generator 2 is then connected -to ground O via
the switching contact 94, so that in this switching position the cathode
voltage is supplied by the high voltage generator 3 whilst the anode
voltage is supplied by the series-connected high voltage generators
1 and 2 together.
In order to measure the tube current in the embodiment shown
in Figure 2, it is not only necessary to extend the high voltage
lO switching device (91, 92, 93, 94) by three further switching contacts
which are not required in the embodiment shown in Figure 1, but the
switching contacts must also be insulated with respect to one another
due to the high voltage supplied by the high voltage generator 2.
This is because in the switching position which is denoted by uninter-
15 rupted lines, the potential on the switching contacts 91, 92 and 94
with the direc-t voltage generated by the high voltage generator 3 is
lower than the potential on t-,he switching contact 93. In the other
switching positions (denoted by broken lines), the potential of the
switching contact 91 is lower than the potential of -the con-tacts 92,
20 93 and 94.
Even though the invention has been described with reference to
an embodiment with a single-phase transformer, it can also be used for
- X-ray generators comprising multi-phase high voltage transformers.
~5
